1. Field of the Invention
This invention relates to a light-emitting diode (LED) troffer, and more particularly to a readily retrofittable LED troffer that adopts LED light sources mounted along two lengthwise sides of an LED module and a reflecting diffuser used to sufficiently mix and uniform light emissions from various LED light sources with consistent intensity and color hue within viewing angles and improved aesthetic perception.
2. Description of the Related Art
Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (with no hazardous materials used), higher efficiency, smaller size, and longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, more energy saving, more efficient correlated color temperature (CCT) tunability, and more aesthetic perception in lighting quality have become especially important and need to be well addressed.
In a retrofit application of a linear LED tube lamp to replace an existing fluorescent tube, the lamp is so configured that the light coming out from the LED light sources illuminates a target area directly. The shortcomings are pixelation, glare, and not enough cut-off at vertical angles greater than 80° above the lamp nadir, which cause users' eyes uncomfortable, thus affecting their mood. Similarly, many conventional LED troffers adopt direct illumination approach and show a poor lighting quality such as hot and dark spots and shadows.
Cree in its design patents, U.S. D667,983 S and D673,711 S, proposes a front-mounted LED approach that uses single linear high-brightness LED array in the middle of the luminaire/troffer (troffer hereafter), shining a reflector and indirectly illuminating a target area. In this case, the back side of the LED mounting surface is thus a heat sink, which faces downward, and the user can see the radiation-like fins of the heat sink in the middle of the troffer. Thus, the design not only looks unaesthetic but shows a dark stripe in the central region. Moreover, because the heat dissipation area of the heat sink is limited, a heat sink with fins must be used to efficiently dissipate the heat generated by operating LEDs, or premature failure of the LEDs occurs. Such kinds of design are expensive because an extra heat sink with heat-dissipation fins is needed. Furthermore, their thermal performance is far from ideal because heat goes up, but the heat sink with fins is in the opposite downward direction, thus affecting convective heat transfer in ambient air.
A conventional 2 by 2 feet panel light troffer uses a square thick acrylic plate as a light diffusing medium. LED light sources located at four lateral sides of the acrylic plate illuminate the four sides of the plate, and evanescent light waves exiting from the front face of the acrylic plate further scatter through a plastic diffuser attached to the acrylic plate in the front panel before launching into a target area. In order to increase optical efficiency, the back panel of the panel light troffer is attached with a reflective sheet. However, such panel light troffers have their light opening flushed with T-bar ceiling grids without recess. Thus, occupants in the room can see the whole bare bright area 2 by 2 feet and feel uncomfortable because a direct glare affects their eyes and distracts them.
In today's lighting applications, correlated color temperature (CCT) tuning is important. Although consumers demand a tunable CCT such as warm-white at 2,700 K, sun-white, natural-white, or cool-white at 6,200±300 K in lighting to help improve the atmosphere in working, exhibiting, or living areas, there have been very few such lighting products in the troffer and luminaire markets. The LED panel light troffers do not have a proper structure to sufficiently perform spatial color mixing, which makes it difficult for them to be successful on the market. Instead, manufacturers can generally make an LED troffer using two kinds of phosphor coated white LEDs, one cool white and the other warm white, to mix the light emissions with different ratios to come up with desired color temperatures. Because at the two color extremes, only one kind of LEDs emits the light, such troffers have poor cost efficiency and luminous efficacy. In spite of these disadvantages, the approach is one of several solutions to changing CCT of an LED troffer in general lighting applications. However, the approach needs a proper color mixing scheme to smooth out lighting outputs such that the color hue is consistent within viewing angles.
Other possible color temperature tuning approaches include a white LED at CCT of 6,200±300 K mixed with an LED having a saturated color, featuring high luminous efficacy; a yellow white LED mixed with a red LED; and RGB color mixing, the earliest approach to varying light color, in which white light is perceived where all three additive primaries overlap. Because of low luminous efficacy and difficulty to meet CIE 1931 recommendations for general lighting in solid state lighting products, such as stabilizing a specific chromaticity over time while LED junction temperatures change from ambient temperature to 120° C. or higher due to different thermal dependencies for an individual LED, the RGB approach is seldom adopted as in general lighting applications today. However, in decorative lighting, RGB color mixing is frequently used. By varying the intensities of the individual red, green, and blue light sources, any colors that human eyes can perceive can be obtained. Surely, in all of the above mentioned CCT tuning approaches, many of same or different LEDs need to be used in combination to achieve a required lumen output. Thus uniformity of the resultant CCT light and color hue within viewing angles becomes an issue if the troffer used cannot provide adequate light averaging and mixing functions.
As LED technologies and standards continue to develop rapidly, today's 2 by 4 troffer requirements of an LED luminous efficacy of 65 lumens per watt and a color rendering index (CRI) of 80 will become unsatisfactory tomorrow to consumers and the Energy Star program. Market also requires minimum lumens emitted from an LED troffer and a specific CCT tolerance for LED chips. For example, today's minimum requirement of 4,000 lumens in a 2 by 4 LED troffer and a CCT tolerance of 175 K may be obsolete tomorrow. Similarly for LED drivers, today's requirements of a power factor of 0.9, a total harmonic distortion (THD) less than 20%, and a power consumption of 50 W may not be good enough tomorrow for energy firms to offer energy rebates, a great incentive for consumers and organizations to adopt LED troffers. In this case, outdated LED modules and LED drivers in LED troffers may need to be removed and replaced with upgraded ones to meet updated consumer needs and new standards. To retrofit a conventional LED troffer for replacing an existing LED driver or LED module, however, is not easy because one must first remove the whole troffer from T-bar ceiling grids. It is especially true when a troffer with a dimension of 2 by 4 feet is quite heavy and when one person alone is less likely to remove such a troffer from at least nine-foot high ceiling.
Emergency lighting is especially important in this consumerism era. The emergency lighting systems in retail sales and assembly areas with an occupancy load of 100 or more are required by codes in many cities. Occupational Safety and Health Administration (OSHA) requires that a building's exit paths be properly and automatically lighted at least ninety minutes of illumination at a minimum of 10.8 lux so that an employee with normal vision can see along the exit route after the building power becomes unavailable. This means that emergency egress lighting must operate reliably and effectively during low visibility evacuations. To ensure reliability and effectiveness of backup lighting, building owners should abide by the National Fire Protection Association's (NFPA) emergency egress light requirements that emphasize performance, operation, power source, and testing. OSHA requires most commercial buildings to adhere to the NFPA standards or a significant fine. Meeting OSHA requirements takes time and investment, but not meeting them could result in fines and even prosecution. If a building has egress lighting problems that constitute code violations, the quickest way to fix is to replace the existing troffer with a multi-function LED troffer that has an emergency light package integrated with the normal lighting. The code also requires the emergency lights be inspected and tested to ensure they are in proper working conditions at all times.
It is, therefore, the manufacturers' responsibility to design a readily retrofittable LED troffer with an emergency lighting package integrated such that after the LED troffer is installed on a ceiling, the LED module can be individually removed from the LED troffer, or the emergency lighting package associated with the LED module can be inspected on site without removing the whole troffer from the ceiling. The retrofittable design can greatly reduce lifetime cost of ownership. Currently no manufacturers have adopted the idea in an architectural troffer used to replace conventional fixtures for fluorescent lamps.
FIGS. 1 and 2 show the design in Cree's design patents, U.S. D667,983 S and D673,711 S. An LED troffer 100 comprises a housing 110 served as a mounting frame and an LED module 140 connected and fixed to the housing 110. In the middle of LED module 140 is a heat sink 141 with fins, which shows a dark stripe area when LED photons emitting from LEDs (not shown) mounted backside of the heat sink 141 are reflected from a back-reflector (not shown) and strike two exit windows 145 and 146 on the two sides of the heat sink 141, making them bright. The LED module 140 is mounted and fixed on top of the housing 110 when the LED troffer 100 is normally installed on T-bar ceiling grids. Thus, the LED module cannot be removed from the bottom side in the service location without first removing the whole LED troffer 100. Furthermore, installing such a troffer on T-bar ceiling grids cannot be done by just one person because it is too heavy and has a dimension of 2 by 4 feet. Installation cost becomes an issue.
FIG. 3 is a typical panel light used in troffer applications. FIG. 4 is an expanded view of a part of the prior art LED panel light troffer in FIG. 3. Referring to FIGS. 3 and 4, an LED panel light troffer 200 comprises a square frame 210 with a light exit window 215 in the central square portion enclosed by the frame 210 and an LED module 220 embedded inside the frame 210. In FIG. 4, the LED module 220 comprises four LED arrays 230 mounted in four sides of the square frame 210. A plurality of LEDs 206 in each of the LED array 230 (only two shown in this corner) are mounted on a plane 90° with respect to the light exit window 215. In the back of the light exit window 215 is a thick acrylic plate, whereas in the further back is a reflective film (not shown). The emitted photons from the LEDs 206 launch into four lateral sides 240 of the thick acrylic plate. Part of photons strike the reflective film, reflect back to the acrylic plate, and exit through the light exit window 215. Rest of photons emit directly from the light exit window at various inclined angles. Because the LED module 220 is embedded inside the frame 210, there is no way to remove the LED module 220 and to replace it without first removing the whole panel light troffer 200 from T-bar ceiling grids and then disassembling it to a component level. Although a square panel light troffer is illustrated here, a rectangular one is also available.